Xylene isomerization

ABSTRACT

Mixtures of C8 aromatic hydrocarbons are contacted in liquid phase with acid zeolite ZSM-5, zeolite ZSM-12 or ZSM-21. The xylene content is thereby isomerized and the ethyl benzene content of the charge is converted in a manner which facilitates separation of the conversion products from the stream of isomerization products.

tatQS t t Aromatics Recycle xylene [1 1 1111 3,850,871 Haag et a1. Dec. 24, 1974 XYLENE ISOMERIZATION 3,691,247 9/1972 Billings 260 668 A 3,751,504 8/1973 Keown et :11. 260/672 T [75] Inventors. Werner 0. Hang, Trenton, David H. 3,756,942 9/1973 Cananach I I I I I I I U mtg/I38 Pennington, both 3,761,389 9/1973 Rollmann 208/64 Assigneez Corporation, New York 3,790,471 2/1974 Arqauer (3! a1. i. 260/672 T N.Y. Primary ExaminerC. Davis [22] Filed: Sept. 13, 1973 Attorney, Agent, or Firm-A. L. Gaboriault [21] Appl. N0.: 397,038

[57] ABSTRACT 52 Us. Cl 260/668 A, 260/672 T Mixtures of 8 aromatic hydrocarbons are contacted 51 um. C1. C076 5/24 in liquid PhaSe with acid Zeolits ZSM-i Zeolite 58 Field 6: Search 260/668 A or The Xylene Content is thereby isomerized and the ethyl benzene content of the [56] References Cited charge is converted in a manner which facilitates sepa- UNITED STATES PATENTS ration of the conversion products from the stream of isomerization products. 2,795,630 6/1957 Lien et a1 260/668 R 3,646,236 2 1972 Keith 612211 260 672 T 2 Claims, 1 Drawin Flgllll'e Porn lene xylen P y YSTOllIZey /0 PATENTEI] [151241974 p-xylene Porn xylene Crystalhzer m, 5 3 H 2 C THYL BENZ C Aromatics NE TOWERV BACKGROUND OF THE INVENTION Xylenes are found in fractions from coal tar distillate, petroleum reformates and pyrolysis liquids in admixture with other compounds of like boiling point. The aromatic components are readily separated from nonaromatics by solvent extraction. Distillation provides a fraction consisting essentially of C aromatics. As will appear below, o-xylene is separable from other Cg aromatics by fractional distillation, and p-xylene is separable by fractional crystallization. Present demand is largely for p-xylene and it has become desirable to convert m-xylene, the principal xylene present in the feed stream, to the more desired p-xylene.

Isomerization of xylenes can be accomplished by the action of any one of a number of known catalysts. It is therefore possible to treat C aromatic fractions in a loop including means to separate desired xylene or xylenes and subject the residue to isomerization with recycle of isomerizate in admixture-with fresh charge. Unless the ethyl benzene be separated from the mixture, it will build up in the loop, reducing capacity of the equipment. Because its boiling point is very close to that of some of the xylenes, separation by distillation is extremely expensive. For that reason, a process which converts ethyl benzene to by-products of reasonably high value and readily separable from xylenes is highly desirable, permitting charge of the entire C aromatics fraction to the loop. For reasons which will ap pear hereinafter, commercial practice of xylene isomerization has used the Octafining Process characterized by an expensive platinum catalyst and hydrogen.

Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of pxylene. See Advances in Petroleum Chemistry and Refining" volume 4, page 433 (lnterscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.

Typically, p-xylene is derived from mixtures of C aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C aromatics in such mixtures and their properties are:

Density Freezing Boiling Lbs./U.S. Point F. Point F. Gal.

Ethyl benzene -l39.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene 54.2 282.4 7.23 O-xylene l3.3 292.0 7.37

Temperature 850F.

wt.% Ethyl benzene 8.5 wt.% para xylene 22.0 wt.% meta xylene 48.0 wt.% ortho xylene 21.5 TOTAL 100.0

An increase in temperature of 50F. will increase the equilibrium concentration of ethyl benzene by about l wt.%, ortho-xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt.%.

Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.

As commercial use of para and ortho xylene has increased there has been interest in isome'rizing the other C aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.

Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C aromatics mixture is fed to such a processing combination in'which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin Cg aromatic feed, the product separation unit performance, and the isomerizer performance.

The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.

Octafiner unit designs recommended by licensors of Octafining usually lie within these specification ranges:

Process Conditions Reactor Pressure l to 225 PSlG Reactor lnlet Temperature Range 830-900F Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr.

Number of Reactors,

Downflow l Catalyst Bed Depth.

Feet H to [5 Pressure Drop. PSI 20 It will be apparent that under recommended design conditions, a considerable volume of hydrogen is introduced with the Cg aromatics. In order to increase throughput, there is great incentive to reduce hydrogen circulation with consequent increase in aging rate of the catalyst. Aging of the catalyst occurs through deposition of carbonaceous materials on the catalyst with need to regenerate by burning off the coke when the activity of the catalyst has decreased to an undesirable level. Typically the recommended design operation will be started up at about 850F. with reaction temperature being increased as needed to maintain desired level of isomerization until reaction temperature reaches about 900F. At that point the isomerizer is taken off stream and regenerated by burning of the coke deposit.

Because of its capability to convert ethyl benzene, Octafining can accept a charge stream which contains that component. Normally, a portion of the ethyl benzene is removed by fractional distillation before the charge is processed. If no attempt is made to reduce ethyl benzene below a few percent by weight, this can be accomplished relatively inexpensively and the ethyl benzene recovered is in usable form as a relatively pure chemical, e.g., for dehydrogenation to styrene.

The Octafiner is in a loop which includes means for separation of desired xylenes; p-xylene by crystallization and, possible, o-xylene by distillation. The C stream stripped of desired xylenes returns to the Octafiner where more of the desired xylenes are generated, for example by isomerization of m-xylene. It will be apparent that ethyl benzene will tend to build up in the loop as other components are removed. The Octafining catalyst has capability for converting ethyl benzene, thus counteracting that tendency. It, the Octafining catalyst, has the disadvantage that it is a hydrocracking catalyst due to the acid function of its silica/alumina base and its content of hydrogenation/dehydrogenation metal of the platinum group. In addition to converting ethyl benzene, this catalyst also causes net loss of xylenes.

Other catalysts have recently been identified as behaving in the same fashion as Octafining catalyst for isomerization of xylenes in C aromatic fractions accompanied by conversion of ethyl benzene. These new catalysts include zeolites of the ZSM- type, zeolite ZSM-l 2 and zeolite ZSM-2 l. ZSM-S type includes zeolite ZSM'S as described in Argauer and Landolt U.S. Pat. No. 3,702,886, dated Nov. 14, 1972 and zeolite ZSM-ll as described in Chu U.S. Pat. No. 3,709,979, dated Jan. 7, 1973 and variants thereon. Zeolite ZSM-l2 is described in German Offenlegungsschrift 2213109. The activity of these catalysts for the stated purpose and ofZSM-2l is described and claimed in copending application of R. A. Morrison, Ser. No. 397,039, filed Sept. 13, 1973, the disclosure of which is hereby incorporated by reference.

In general. Octafining catalyst and the zeolite catalysts referred to above behave in about the same manner, except for their aging characteristics; decline of activity with time on stream.

A typical charge to the isomerizing reactor (effluent of crystallizer for separation of p-xylene) may contain 17 wt.% ethyl benzene, 65 wt.% m-xylene, 11 wt.% pxylene and 7 wt.% o-xyleneThe thermodynamic equilibrium varies slightly with temperature in a system in which o-xylene is separated in the loop by fractional distillation prior to the crystallizer. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.

ln Octafining, ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.

The rate of ethyl benzene approach to equilibrium concentration in a C aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect onethyl benzene approach to equilibrium, Temperature change within the range of Octafining conditions (830 to 900F.) has but a very small effect on ethyl benzene approach to equilibrium.

Concurrent loss of ethyl benzene to other molecular weight products relates to approach to equilibrium. Products formed from ethyl benzene include C naphthenes, benzene from cracking, benzene and C aromatics from disproportionation, and total loss to other than C molecular weight. C and lighter hydrocarbon by-products are also formed.

The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.

Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C aromatics and C and lighter hydrocracking products.

Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity of Octafining catalyst and this effect is proportional to its concentration in a C aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.

A more recent development than Octafining is Low Temperature lsomerization (LTI) as described in Wise U.S. Pat. No. 3,377,400, dated Apr. 9, 1968. That process is particularly effective when the zeolite catalyst employed is ZSM-4 as described in Bowes et al. U.S. Pat. No. 3,578,723, dated May ll, l97l.

The advantages of LTl rest in its capability to isomerize xylenes in liquid phase at relatively low temperatures and the lack of necessity for hydrogen pressure in the reactor. The zeolite catalyst, particularly ZSM-4, ages very slowly even without hydrogen or a hydrogenation/dehydrogenation metal component on the catalyst. However, LTl has one disadvantage. It leaves ethyl benzene unchanged.

Because of the ethyl benzene content of C aromatic fractions is unchanged in LTl operations as practical heretofore, such operations incur severe costs in capital investment and in operating expense to dispose of the ethyl benzene in order that it shall not build up in the system. Because of the minor difference in boiling point between ethyl benzene and certain of the xylenes, complete removal of ethyl benzene from the charge is prohibitive in cost. The practical way to handle this component is to provide an additional distillation column in the loop to remove ethyl benzene at substantial cost.

It has now been found that catalysts which are the acid forms of zeolite ZSM-S type, zeolite ZSM-l2 or zeolite ZSM-Zl provide C aromatic isomerization catalysts which not only are very active and selective for shifting methyl groups on xylenes, but also convert ethyl benzene in a manner not previously described as effective for feeds of this type. The process is conducted in liquid phase at the upper part of the ranges described in the cited patents for practice of LTI. At these conditions, the catalyst of this invention induce extensive disproportionation of ethyl benzene with little disproportionation of xylenes.

It is, accordingly, a primary object of this invention to isomerize the xylene content of C aromatic fractions, but also to convert the ethyl benzene content thereof in-a novel and unexpected fashion.

As will be readily understood, the type of ethyl benzene conversion typical of Octafining cannot occur when hydrogen is not supplied to the reactor. It will be recalled that the conversion of ethyl benzene in Octafining proceeds by hydrogenation to ethyl cyclohexane which undergoes rearrangement to dimethyl cyclohexane. Dehydrogenation of that compound yields xylenes.

Despite the fact that the classical (Octafining) type of ethyl benzene conversion is impossible under the conditions characteristic of this invention, the conversion of that compound in the process of the invention does result in enhanced yield of desired aromatic compounds, primarily benzene. The polyethyl benzenes resultant from disproportionation of ethyl benzene may be subjected to transalkylation with benzene for production of ethyl benzene.

The above objects are attained in a process illustrated in the single FIGURE ofdrawings which is a diagrammatic representation of apparatus suited to practice of the invention.

A mixture of C aromatics is supplied to the system by line I, as from solvent extraction of a narrow cut taken by distillation of product of reforming a petroleum naphtha over platinum on alumina catalyst in the presence of hydrogen. The feed passes to distillation in ethyl benzene tower 2, from which a portion of the ethyl benzene content is taken overhead by line 3. It is impracticably expensive to attempt removal of substantially all of the ethyl benzene by tower 2. The amount removed as essentially pure ethyl benzene, suitable for charge to such operations as dehydrogenation to styrene, will depend on exact nature of the charge and demand for different products. Optionally, the ethyl benzene tower 2 can be omitted with further reduction in investment and operating costs.

Bottoms from tower 2 are constituted by the xylenes present in the charge and a reduced content of ethyl benzene. This mixture passes by line 4i and is blended with recycle xylenes, derived in a manner presently to be described, from line 5. The blended stream is admitted to splitter tower 6 from which a heavy end is withdrawn by line 7. In the embodiment shown, that heavy end 'is constituted by C aromatics derived from disproportion of ethyl benzene and from the minor side reaction of transalkylation of xylenes in the isomerizer. Alternatively, when it is desired to recover o-xylene as a separate product, splitter tower 6 may be operated to include o-xylene in the bottoms which are then passed to distillation for separation of o-xylene from (3 aromatics (not shown). l

The overhead of splitter tower 6 passes by line 8 to means for separation of p-xylene. In the embodiment illustrated, p-xylene is separated by fractional crystallization in crystallizer 9, involving chilling and filtration of p-xylene crystals from the liquid phase, for example. in the manner described by Machell et al. U.S. Pat. No. 3,622,0l3, dated May 9, 1972. It will be understood that other systems for p-xylene separation may be used in a plant for practice of this invention, e.g., selective sorption as described in Cattanach US. Pat. No. 3,699,182 dated Oct. 17, 1972. By whatever means separated, high purity p-xylene is withdrawn as product by line It).

The stream of C aromatics of reduced p-xylene content is withdrawn from crystallizer 9 by line 11, passed through heater l2 and admitted to catalytic isomerizer 13 where it is contacted at reaction conditions with the acid form of ZSM-5 type zeolite or zeolite Z SM-l2 or zeolite ZSM-2l. The principal reaction in isomerizer I3 is shifting of methyl groups in xylene molecules toward the equilibruim concentrations of the three xylens. In addition to xylene isomerization, secondary reactions of transalkylation occur to produce benzene, toluene, polyethyl benzenes, polymethyl benzenes, ethyl toluenes and ethyl xylenes. Important for the purposes of this invention is disproportionation of ethyl benzene to yield benzene and polyethyl benzenes.

The isomerizate produced in isomerizer 13 is transferred by line 14 through heat exchanger 15 to stripper 16. The light ends of the isomerizate (benzene, toluene and normally gaseous hydrocarbons) are taken overhead byline 117 from stripper l6 and the balance passes by line 5 to be blended with fresh feed and recycled in the process.

The catalyst is prepared by converting the zeolite to ,acid form" by calcination which converts tetraalkylammonium cations characteristic of these zeolites to protons by decomposition of the substituted ammonium cations. Additional protons and various metal cations may be substituted for the sodium cations present in the zeolites as formed by base exchange in conventional manner. It is csential to success in the present process that the zeolite catalyst be at least partially in the acid form, that is, that at least a portion of the cation positions be occupied by protons. Metal cations of various types may occupy the other sites if desired.

Since the process is conducted in the absence of added hydrogen, there is no need for metals of the transition groups such as nickel, platinum, palladium, etc. These metals may be present, but as now understood, the process appears to be unaffected by such cations.

The zeolite crystals are preferably embedded in a bonding material to provide pellets of desired size and resistance to attrition. A suitable binder is alumina. In 1 order to provide a preponderance of the active zeolite, the binder is a minor constituent of the composite. A particularly preferred catalyst is constituted by pellets of 35 wt.% alumina and wt.% of the acid form of type ZSM-5 zeolite ZSM-l2 or zeolite ZSM-Zl.

The isomerization process of this invention is operated in the liquid phase at temperatures of 500F. to 660F. under pressure sufficient to liquefy the charge. Aside from the need to maintain liquid phase conditions, pressure does not seem to be a critical parameter and will be dictated in the usual case by economic and engineering considerations. Excessively high pressures, above about 1000 p.s.i.g. will be generally undesirable.

though fully operative, because of the great strength of reaction vessel walls required at high pressures, making the equipment unnecessarily expensive and requiring expensive compression stages.

Space velocities will vary in the range of 0.5 to volumes of charge per volume of catalyst per hour (liquid hourly space velocity, LHSV). In general, temperature and LHSV will be coordinated to provide a desired severity which will provide an adequate degree of xylene isomerization and ethyl benzene conversion without excessive losses to by-products. Thus, temperatures in the lower part ofthe temperature range will normally call for low space velocities. The process of the invention is essentially an improvement on the low temperature process (LTI) which itself has several advantages over Octafining: e.g. no hydrogen gas is used avoiding the use of recycle compressors and gas separators; no non-aromatic by-products are formed such as light gases and naphthenes that are difficult to separate; operation in the liquid phase rather than gas phase requires less capital investment; isomerization occurs with higher selectivity. Ethyl benzene is not converted and is essentially an inert diluent. With feeds containing ethyl benzene, means have to be provided for its removal, otherwise it would build up in the recycle stream. For example, ethyl benzene is removed by distillation in a special ethyl benzene column.

According to this invention, it is possible to convert ethyl benzene during xylene isomerization by operating the liquid phase isomerization under more severe conditions than would otherwise be required for selective isomerization using specific zeolite catalysts. This modified LTI process retains the process simplicity of the convensional LTI process and converts ethyl benzene without the need for additional process equipment.

The proposal is based on the surprising finding that with ZSM-S type, with ZSM-l2 and with ZSM-2l as catalysts, disproportionation and transalkylation occur very selectively to convert ethyl benzene relative to xylenes.

For example, we found the following rate constants for the disproportionation reactions at 550-600F.;

Rel. rate constants 2 ethyl benzene-s benzene diethyl benzene I 2 xylencs toluene trimethyl benzene l Transalkylations also occur with the rate constants shown:

Rel. rate constants (3) ethyl benzene xylene benzene ethyl xylene 16.8

(4) xylene ethyl benzene-toluene ethyl toluene 3.6

feed. For typical feeds containing 10-25% ethyl benzene it is found that Percent ethyl benzene conversion :8-20. Percent xylene conversion The term transalkylation, i.e. transfer of alkyl groups between aromatic hydrocarbons can be used to include disproportionation (transalkylation between like molecules).

The only significant products resulting from ethyl benzene and xylene conversion to other compounds than isomers are aromatics with 6, 7, 9 and 10 carbon numbers. They can be recovered for chemical use (e.g. benzene) or used as valuable high octane components for motor fuels.

The severity of the process conditions should be optimally chosen such that sufficient ethyl benzene is converted to prevent its buildup in the recycle stream (see FIGURE 1 attached). This optimum amount of conversion depends on the feed composition, on the particular method to remove p-xylene (e.g. crystallization or extraction via sorption) and whether o-xylene is also recovered for sales. In general, the following equation describes the degree of ethyl benzene conversion:

AEB R X AXYL.

xylenes ethyl benzene Fresh Feed 20 p-xylene extracted 18.4 xylenes transalkylated 1.3 total xylenes removed:

xyl l9.7 ethyl benzene conversion required AEB 0.25 X 19.7 4,92

The necessary degree of ethyl benzene conversion is controlled by the severity of the process conditions, in particular by temperature and space velocity that can be traded off within relatively wide limits. Typical conditions are 500660F., suffient pressure to establish liquid phase conditions, i.e., -520 p.s.i.g. respectively, or higher, a LHSV 0.5-10, preferably 1-5, catalysts are HZSM-S type, HZSM-l2 and HZSM-Zl.

The ratio of ethyl benzene to xylene (R) in the recycle stream can be the same as that in the fresh feed. However, it has been found that the small loss of xylenes due to transalkylation can be further minimized and a higher ultimate yield of p-xylene can be obtained 0 by having a higher ratio R in the recycle stream than in the fresh feed. This can be achieved during plant startup by adding some ethyl benzene to the feed for an initial period of several hours; alternatively, start-up conditions of lower severity can be chosen that lead to the build-up of ethyl benzene in the recycle stream to the desired level. This modified procedure is particularly preferred with a feed that is low in ethyl benzene. The

following table illustrates the benefit of this procedure:

' binder, activated in 6cc/min. dry air flow per cc of catalyst from 25C. to 538C. followed by 3 hours at R in Fresh Feed 01H H OJ H 538C.) The'mixed charge containing EB had composi- R in Recycle Stream 0,111 0.2 0,483 tron 16.2% EB, 61.2% m-xylene and 22.6% o-xylene Y mg; of 92-4 97-0 9&4 5 (m-xyl/o-xy1= 2.7). Table I reports the detailed prodxylenes fed) 1101 31121137885. ethyl w Xylem) Xylene loss is that portion of the xylenes that undergoes transalkylation, mostly to C and C aromatics. EXAMPLE 1 This example concerns isomerization of a mixed C,,- 10 EXAMPLE 2 aromatics feed that closely resembles a commercial HZSM-ZI is also an excellent Xylene isomerization isomerization f ed stock, whi h i ll i h i catalyst with activity, selectivity and aging characterisxylene and contains ethyl benzene (EB). In addition to t Similar t t e ou d fo HZSM5. With a feed the activity and selectivity data obtained using this containing 16 p rcen ethyl benzene, 62% m-xylene mixed feed, data are given concerning the effect of and 22% -Xy qu ium on erSiOn to pethyl benzene on selectivity and activity, and the cata- Xylene Was Obtained at 48001: W'HSV 4, 400 pl aging m (liquid phase). Xylene loss due to disproportionation The catalyst is HZSM-S zeolite, 35% A1 0 was less than TABLE I XYLENE ISOMERIZATION OVER ZSM-S (35% A1 0 BINDER) PRODUCT ANALYSIS (W'1".%)

TIME ET.

TEMP. of XYL. ET. BENZ. (HRS.) (F.) BENZ. TOL. BENZ. p-xyl. m-xy1. o'xyl. Cg+ EQUIL. LOSSWZ) LOSS(7() Catalyst: DJK-ZZZ-Z-QB. HZSM-S (35% Al,0 binder). Feed: 16.2% 158.612)? m-xyl and 22.6% O-xyL. WHSV 4 (based on zcolite component). pressure 600 p.s.i.g.

' I! olequi1.= (Normalized tut/71 p-xy1/0.239) X 100 Relative percentage losses.

' Normalized wt. percentages in parenthesis.

HZSM-Zl, having a siO /Al O ratio of 29 was used as the catalyst. Before use, the material was calcined in air 1C/min. from room temperature to 1000F. and held at l()F. for hours. The C aromatic feed con- TABLE ll XYLENE ISOMERIZATION OVER HZSM-Zl PRODUCT ANALYSES (WT.%)

x)(1..'- 15.13:- TIME TEMP. 7 0F" LOSS LOSS (HR) F, YWHSV BENZ. TOL. EB -X 1." m-xyl. 6-x 1.' c.,+ 120011.. (WT.7r) (WT/2) Catalyst: HZSM-Zl KS-6299. Feed: [5.5% EB, 62.0% m-xyl and 22.5% o-xyl. Pressure 400 lbs.

, "Normalized weight percentages in parentheses.

"Values in parentheses estimated. I X of equilibrium (Normalized wt p-xyl./0.239) I00. Relative wtfk loss due to disproportionation and transalkylation.

We claim:

1. In a process for conversion of a mixture of aromatic compounds having 8 carbon atoms, said mixture containing ethyl benzene and xylenes, to isomerize xylenes contained in said mixture and convert at least part of ethyl benzene so contained to compounds readily separable by distillation from 8 carbon atom aromatics; the improvement which comprises containing such mixture of eight carbon atom aromatic compounds with a catalyst which comprises acid zeolite of stream of C aromatics containing xylenes and ethyl benzene which comprises mixing said feed stream with recycle xylenes derived as hereinafter recited, separating p-xylene from the mixture to provide product pxylene and a C aromatic stream lean in p-xylene, subjecting said lean stream to contact with a catalyst which comprises acid zeolite of the ZSM-S type, acid zeolite ZSM-l2 or acid zeolite ZSM-2l in liquid phase and in the absence of added hydrogen at a temperature of about 500F. to about 660F., recycling the product of said contacting as recycle xylenes, mixing as aforesaid to provide a p-xylene recovery loop, and maintaining in said loop a concentration of ethyl benzene greater than the concentration of ethyl benzene in said feed stream.

PATENT OFFICE EilATEi ()F CORRECTION PM'ER 2 NO. 3, 56, 71

LL-RE'EL 1 December 24, 1974 i-VE "'3K\5 WERNER O. HAAG and DAVID H. OLSON =n tlw above-identified patent and that send Letters Patent I 4;; her con'zzctej as sham b iw Column 11, line 48, "containing" should be --contacting Engncd and Sealed thus twenty-second Day Of July 1975 [SEAL] Arrest:

RUTH c. MASON c. MARSHALL DANN Atlvsling Officer Commissioner of Parents and Trademarks 

1. IN A PROCESS FOR CONVERSION OF A MIXTURE OF AROMATIC COMPOUNDS HAVING 8 CARBON ATOMS, SAID MIXTURE CONTAINING ETHYL BENZENE AND XYLENES TO ISOMERIZE XYLENES CONTAINED IN SAID MIXTURE AND CONVERT AT LEAST PART OF ETHYL BENZENE SO CONTAINED TO COMPOUNDS READILY SEPARABLE BY DISTILLATION FROM 8 CARBON ATOM AROMATICS; THE IMPROVEMENT WHICH COMPRISES CONTAINING SUCH MIXTURE OF EIGHT CARBON ATOM AROMATIC COMPOUNDS WITH A CATALYST WHICH COMPRISES ACID ZEOLITE OF THE ZSM-5 TYPE, ACID ZEOLITE ZSM-12 OR ACID ZEOLITE ZSM-21 IN LIQUID PHASE AND IN THE ABSENCE OF ADDED HYDROGEN AT A TEMPERATURE OF ABOUT 500*F. TO ABOUT 660*F.
 2. A process for manufacture of p-xylene from a feed stream of C8 aromatics containing xylenes and ethyl benzene which coMprises mixing said feed stream with recycle xylenes derived as hereinafter recited, separating p-xylene from the mixture to provide product p-xylene and a C8 aromatic stream lean in p-xylene, subjecting said lean stream to contact with a catalyst which comprises acid zeolite of the ZSM-5 type, acid zeolite ZSM-12 or acid zeolite ZSM-21 in liquid phase and in the absence of added hydrogen at a temperature of about 500*F. to about 660*F., recycling the product of said contacting as recycle xylenes, mixing as aforesaid to provide a p-xylene recovery loop, and maintaining in said loop a concentration of ethyl benzene greater than the concentration of ethyl benzene in said feed stream. 